The capacity of an elevator system, which is defined during the design phase of creating a new building, must serve the elevator traffic in the building in dependence upon the nature of expected building usage. Things that can result in adequate service with fewer elevator shafts include the manner of handling up-peak rush traffic, and the manner of handling down-peak rush traffic, along with the expected level of interfloor traffic (neither primarily up nor down and primarily not involving a lobby). Buildings which intend to serve multi-floor tenants, particularly high-density multi-floor tenants, are expected to have a high degree of interfloor traffic. In larger buildings, significant interfloor traffic may occur contemporaneously with the up-peak rush traffic from the lobby, the down-peak rush traffic to the lobby, as well as the noon traffic lobby down-peak and up-peak.
In extremely tall buildings (for instance, over 80 stories), significant interfloor traffic at the high end of the building may be served by a substantially complete elevator system, which provides no service at all below a certain floor. Access to and from the ground levels (e.g., the lobby) is by means of totally separate shuttle elevators, the upper end of which is designated as a lobby floor, referred to sometimes as a "sky lobby". However, the use of a sky lobby is inconsistent with smooth flowing traffic, and is found to be objectionable by passengers in other than the very tallest buildings.
In many instances, when the best dispatching techniques are employed, with elegant up-peak rush dispatching modes and elegant down-peak rush dispatching modes, there nonetheless arises a situation in which there is significant interfloor traffic in the highest end of the building, which alone results in the need for additional high rise elevators. Typically, high rise elevators only serve a group of contiguous upper floors, with a non-service express zone reaching to the ground levels.